Based on the FAQ from their website, there will be bridging bus service to Stadium MRT
https://www.marinarun.com.sg/faq
Free shuttle bus service from Stadium MRT station CC6 to Gardens by the Bay East at Rhu Cross <-----> (Return shuttle service between Stadium MRT & Gardens by the Bay East will run from 3:30pm to 11:30pm (on a first come first serve basis).

The race time for 21km/30km is now earlier at 6pm 👍. The previous (2018) starting time was 7:30pm.
Let's say if run at the worse timing of 4.5hrs (cut off time), still can finish the run by 10:30pm. So it is still not too late if need to catch bus/train home.
Should be good enough reasoning to sign up, unless those negative feedbacks (poorly lit route/run out of finishing medal/run out of finisher t-shirt size, etc) from previous Marina Run really put you off.